1. Field Of The Invention
This invention relates to a process for preparing a magnetic fluid, and more particularly to a process for preparing a magnetic fluid having an improved saturation magnetization by stably dispersing fine particles of ferrite in a carrier liquid which vapor pressure is low at a high concentration.
2. Description Of The Prior Art
Fine particles of ferrite are prepared by pulverization, co-precipitation, vapor deposition, etc., among which co-precipitation procedure is usually used from the viewpoint of purity, particle size control, productivity, etc. However, co-precipitation is a precipitation reaction from an aqueous solution containing iron ions, and thus the resulting fine magnetic particles are in a suspended state in an aqueous solution.
On the other hand, it is desirable that fine particles of a magnetic material for a magnetic fluid are not in a coagulated state, but in an individually dispersed state. Thus, in case of fine particles of a magnetic material prepared by co-precipitation it is necessary to adsorb a surfactant for preventing a coagulation or agglomeration onto the surfaces of fine particles in a dispersed liquid state without passing through a drying step having a risk of coaguation and agglomeration of the fine particles themselves. Therefore, an aqueous surfactant is used.
A carrier liquid for a magnetic fluid containing dispersed fine particles which are coated with a aqueous surfactant is restricted to a relatively volatile solvent such as kerosine, toluene, etc., and when the magnetic fluid is used as a magnetic fluid seal, a magnetic fluid polishing, etc., evaporation of the carrier liquid is a problem so important as to deteriorate the function of the magnetic fluid itself.
Generally, a magnetic fluid is a dispersion of fine particles of ferrite in a carrier liquid by using a dispersing agent such as a higher fatty acid salt, sorbitan ester, etc. However, any high stability of dispersion cannot be obtained from a mere dispersion of fine particles of ferrite in a carrier liquid and such a dispersion is not used in practical applications.
Thus, the present inventors have previously proposed a process which comprises first dispersing fine particles of ferrite coated aqueous surfactant in a hydrocarbon solvent waving a low boiling point, then mixing the dispersion with a carrier liquid, and removing the hydrocarbon solvent therefrom by distillation under ultrasonic irradiation, thereby substituting the liquid carrier which is employed in ferrofluid (Japanese Patent Application Kokai (Laid-open) No. 63-3099).